Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due to its high octane rating.
Industrial alkylation processes have historically used Bronsted acid catalysts such as hydrofluoric or sulfuric acid under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid.
Bronsted acid catalyzed isoparaffin:olefin alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986). For a discussion of the safety and environmental concerns associated with liquid Bronsted acid alkylation, see U.S. Pat. Nos. 4,938,935 and 4,985,220 to Audeh and Greco, as well as U.S. Pat. No. 4,938,936 to Yan, which teach methods for containing and/or neutralizing HF acid clouds following accidental releases.
Various hydrofluoric acid catalyst complexes have, in the past, been disclosed as useful for various purposes. However, the prior art has not contemplated the novel catalyst complex and alkylation process of the present invention which improves safety and avoids the environmental hazards associated with HF alkylation processes.
U.S. Pat. No. 2,615,908 to McCaulay teaches thioether-HF-copper complex compounds and a method for preparing the same. Potential uses for the thioether-HF-copper complex compounds are listed from column 6, line 55 through column 8 at line 3. The method is said to be useful for purifying HF-containing vent gases from an industrial HF alkylation plant. See column 7, lines 10-24.
U.S. Pat. No. 3,531,546 to Hervert discloses a HF-CO.sub.2 catalyst complex which is said to be useful for alkylation as well as olefin isomerization.
U.S. Pat. No. 3,795,712 to Torck et al. relates to acid catalysts comprising a Lewis acid, a Bronsted acid, and a sulfone of the formula R-SO.sub.2 -R', where R and R' are each separately a monovalent radical containing from 1 to 8 carbon atoms or form together a divalent radical having from 3 to 12 carbon atoms. By dissolving the acid in a sulfone, the effective acid strength is said to be markedly increased.
U.S. Pat. No. 3,856,764 to Throckmorton et al. teaches an olefin polymerization catalyst comprising (1) at least one organoaluminum compound, (2) at least one nickel compound selected from the class consisting of nickel salts of carboxylic acids, organic complex compounds of nickel, or nickel tetracarbonyl and (3) at least one hydrogen fluoride complex prepared by complexing hydrogen fluoride with a member of the class consisting of ketones, ethers, esters, alcohols, nitriles, and water.
U.S. Pat. No. 4,646,488 discloses an anhydrous nonalcoholic alkylation catalyst comprising a mixture of a mineral acid and an ether in proportion of from about 50 to about 99 weight percent of mineral acid and from about 1 to about 50 weight percent of ether. Useful mineral acids includes HF; see column 4 at lines 56-60.
Recently, research efforts have focused on Lewis acid promoted solids as alkylation catalysts. The term "Lewis acid" as used herein refers to a molecule which is capable of combining with another molecule or ion by forming a covalent chemical bond with two electrons from the second molecule or ion. Examples of Lewis acids include boron trifluoride (BF.sub.3), boron trichloride (BCl.sub.3), antimony pentafluoride (SbF.sub.5), and aluminum chloride (AlCl.sub.3). For a general discussion of Lewis acids, see Friedel-Crafts and Related Reactions, Interscience Publishers, Chapters III and IV (1963), which is incorporated herein by reference. The following U.S. Patents address alkylation in the presence of a Lewis acid-promoted solid and are incorporated by reference as if set forth at length herein.
U.S. Pat. No. 4,918,255 to Chou et al. teaches a process for alkylating an isoparaffin with an olefin in the presence of a catalyst complex comprising a Lewis acid and a large pore zeolite and/or a non-zeolitic inorganic solid, together with a controlled amount of water.
U.S. Pat. No. 4,935,577 to Huss, Jr. et al. discloses a catalytic distillation alkylation process employing a Lewis acid promoted solid catalyst comprising a non-zeolitic inorganic oxide, a large pore crystalline molecular sieve, and/or an ion exchange resin.
U.S. Pat. No. 4,992,616 to Chou et al. teaches catalytic isoparaffin:olefin alkylation in the presence of added water, a bound or unbound large pore zeolite, and a Lewis acid, wherein the Lewis acid is present in an amount exceeding that required to saturate the zeolite and its binder or matrix, if present.
The two-part article, "Modern Alkylation", by Lyle F. Albright, Oil and Gas Journal, Nov. 12 and 26, 1990, summarizes the state of the art in alkylation technology, and highlights problems associated with liquid Bronsted acid catalysts such as HF and H.sub.2 SO.sub.4, and notes safety and environmental concerns associated with using and storing substantial quantities of these acids, which concerns underscore the desirability of developing a commercially viable low acid inventory isoparaffin:olefin alkylation process.
Various techniques have been explored for improving contact between a Bronsted acid catalyst and hydrocarbon reactants. For example, U.S. Pat. No. 3,780,130 describes a gas-fog alkylation process, wherein a fog or mist of acid is allowed to react with the hydrocarbon. Although this prior art noted some improvement in the alkylation process, very high voltages (up to 4000 v) are required for the generation of acid fog making the process impractical on a commercial scale.
In the process described in U.S. Pat. No. 2,380,234, a small amount of solid is dispersed in the acid phase. The resultant system shows limited improvement in the alkylation efficiency as shown by a slight increase in the alkylate yield.
More recently, U.S. Pat. No. 4,783,567, teaches a process wherein the hydrocarbon feed is contacted with hydrofluoric acid in a reactor with a fixed bed of a solid packing. The reference reported minor improvements associated with the use of solid packings.
Clearly then, it would be desirable to provide a catalyst system and alkylation process which not only produces high quality alkylate but also avoids the safety and environmental problems associated with Bronsted acid-catalyzed alkylation.